1. Field of the Invention
The present invention relates generally to the field of organic-inorganic hybrid composites prepared by sol-gel chemistry for use as dental restorative material or as a bone substitute for bone repair.
2. Background Information
Composites have been used as dental restorative materials since 1962. Bioceramics, such as hydroxyapatite are widely used as bone substitutes; however, they are brittle in nature. Composites described in the present invention combine the properties of polymers and that of the ceramics to provide the necessary strength and toughness. Though the formulation of the composite materials have changed in the last three decades, the underlying chemical process involved has not changed. An acrylate monomer is used as the source of resin, and silica is used as the filler material. Though these composites have superior aesthetic qualities, there are still problems with the mechanical properties such as toughness and wear resistance, problems with adhesion to the dentin of the tooth, and problems with shrinkage of the composite material once it has been placed in the tooth and is cured. These problems are more of an issue when the composite is used for posterior restoration.
Durability is a major problem with posterior composite restoration material. Life spans of large fillings are usually fewer than five years, which can be attributed to inadequate resistance to wear of composites under masticatory friction. The insufficient interaction between the reinforcing filler and the resin binder may be responsible for the lack of wear resistance of these composite materials. It has been demonstrated that an ultra fine compact filled composite has a Young's modulus higher than that of dentin. These composites also have good Vickers hardness and high compressive strength. This improvement and the mechanical properties of the ultra fine compact filled composite may be the result of better binding of filler particles with the resin.
Two of the other major problems associated with the present day dental restorative materials are lack of adhesion to the dentin and shrinkage of the resin during polymerization. The lack of adhesion results in micro leakage and formation of secondary cracks along the interface between the tooth and the restorative material. The shrinkage often occurs during the conversion of monomer to polymer and works against the formation of an adhesive bond between the resin and the dentin of the tooth. The new fourth generation composite materials use a hydrophilic primer which can penetrate into the dentin and produce enhanced adhesion of the composite material. Conditioning of the dentin of the tooth and the use of hydroxy ethyl methacrylate (HEMA) as a primer results in sheer bond strengths of 17-20 megaPascal (MPa). Even though the use of these materials and the conditioning of the dentin increases sheer bond strengths to acceptable levels, shrinkage is often a problem.
The shrinkage of commercially available filled composite resin ranges from 2.6%-7.1%. These shrinkage values differ as a result of their monomer composition, various degrees of polymerization, filler type, and filler concentration. Use of oxaspiro monomers have been considered as the precursor for the resin, but these monomers expand 3.5%-3.9% in volume under polymerization conditions, which is not acceptable.
The properties of a composite material are greatly influenced by the degree of mixing between the inorganic (filler) and the organic (resin) phases. In a molecularly tailored system, an organically modified ceramic precursor will result in the synchronous formation of inorganic (silica) and organic (resin) components. Such a composite could be obtained by using sol-gel chemistry. The mixing of the inorganic and organic matrix in these composites is at the molecular level, and the particles are often nanometer sized. As expected, these hybrid nano-composite materials have toughness three orders of magnitude higher than the ceramic alone. Depending upon the morphology, phase behavior and organic-inorganic ratios, these composite materials comprise a continuum ranging from glass reinforced organic polymer to polymer modified glass.
Composites obtained using conventional sol-gel chemistry suffer from shrinkage problems as a result of evaporation of excess solvents and water. Ellsworth et al. (Chem. Mater. 5:839-844 (1993)) disclose the synthesis of non-shrinking sol-gel composites with higher glass content using a modified esterification process for synthesizing poly-silicic acid esters with unsaturated alcohols. This method eliminates most of the shrinkage problems and long drying times required with conventional composites. However, the materials produced according to these methods are solids which are not particularly useful as dental restorative material, which requires a liquid to gum-like consistency. Additionally, these materials will have problems adhering to the dentin, and thus do not address the adhesion problems of composites.
Goodwin and Kenney in Inorg. Chem. 29:1216-1220 (1990) disclose a method of converting a silicate to an alkoxysiloxane using very simple alcohols, such as methanol, butanol, and ethanol in the presence of hydrochloric acid. No transformation of these compounds to composites is disclosed. Further, there is no suggestion to select and use alcohols containing particular functional groups responsible for imparting improved properties to ceramics produced from these compounds.
U.S. Pat. No. 4,381,918 to Ehrnford discloses a method of producing a dental restorative composite of organic resin and inorganic porous particles; however, the composite is not a "true" organic-inorganic composite. Sol-gel chemistry is not used to obtain this composite.
Other patents disclose processes of making organic-inorganic hybrid polymers, which are not disclosed to be useful as dental restorative materials. U.S. Pat. No. 5,064,877 to Nass et al. disclose a process of fixing inorganic species in an organic matrix. Monomeric compounds were reacted with complexing agents which have polymerizable functional groups. Hydrolysis and condensation took place prior to polymerization and polycondensation. The concentration of inorganic species in the matrix was small. U.S. Pat. No. 5,231,156 discloses the preparation of organic-inorganic hybrid polymers. A 5%-25% R.sub.x Si(OR').sub.4-x was mixed with 95%-75% of an organic monomer capable of a polymerization reaction with the R group.
A good dental material should possess a long lifetime, high toughness or durability, high abrasion resistance, superior adhesion to dentin, approximately 0% shrinkage/expansion, good esthetics, and comparable radiopacity to the enamel. At the present time, there are no dental restorative materials available which have all of these characteristics. Thus, there is a need in the dental field for a restorative material which has all of these characteristics.
The present invention provides a unique approach to producing a hybrid composite for dental restorative purposes or as a bone substitute. By selecting specific monomers (alcohols) having desired functional groups, alkoxides (single source precursors) containing these functional groups are obtained via esterification, and then transformed to hybrid composites having advantageous properties. These hybrid composites possess good adhesiveness attributable to the presence of the functional groups of the monomer which during the method are transferred by esterification to the single source precursor. Polynuclear alkoxides of silicon, aluminum, titanium, or zirconium can be obtained which will function as single source precursors for organic-inorganic hybrid composites of the present invention. These single source precursors can also contain mixtures of silicon, aluminum, titanium, and zirconium according to the present invention which are transformed to obtain mixed organic-inorganic hybrid composites. None of the prior art organic-inorganic composites possess the properties of low to no shrinkage, and good adhesion. As a result of being produced from a single source precursor, the components of the composite of the present invention are consistently mixed each time, whereas the prior art methods may result in the lack of uniform consistency due to mixing of components within a single preparation or among multiple preparations. None of the prior art discloses the use of a single source precursor containing the functional groups which result in enhanced properties of the resultant hybrid organic-inorganic composites.
The hybrid composites of the present invention consist of a net work of very fine, probably nanometer sized SiO.sub.2 network, which is penetrated by acrylate based polymers. Because of the very small size of the SiO.sub.2 particles, the composites of the present invention are transparent and translucent making them good candidates, particularly for dental restorative materials.